1. Field of the Invention
The present invention relates to a hydrostatic vehicle driving system applicable to a working vehicle like that having a mower unit, namely, a lawn tractor, including a hydrostatic transmission preferably incorporated in a transaxle apparatus, wherein the speed reduction ratio exerted by the driving system is automatically changed in correspondence to the condition of load on an engine.
2. Related Art
Conventionally, it is well-known that a working vehicle like a lawn tractor having a mower unit is equipped with a belt type stepless transmission having a split pulley (hereinafter, xe2x80x9ca belt transmissionxe2x80x9d) for avoiding overload on an engine of the vehicle. Generally, such a belt transmission is interposed between the output shaft of the engine and the input shaft of the transaxle apparatus incorporating a mechanical transmission like a multi-speed gear type transmission. The belt transmission automatically changes its speed reduction ratio according to the change of output rotary speed of the engine. In this regard, the speed reduction ratio of the belt transmission is smaller when the engine drives fast, and greater when the engine drives slowly.
This system is advantageous in that, while the engine drives fast, the smaller speed reduction ratio is established for efficient exertion of the capacity of the engine, and that, while the engine drives slow, the engine can be automatically prevented from overload because of the greater speed reduction. However, the tandem arrangement of the belt transmission and the transaxle apparatus interferes with minimization and cost-saving of the vehicle.
On the other hand, for being applied to a working vehicle such as a lawn tractor, there is a well-known conventional transaxle apparatus (an integral hydrostatic transmission, namely, an xe2x80x9cIHTxe2x80x9d) which incorporates a hydrostatic stepless transmission (hereinafter, an xe2x80x9cHSTxe2x80x9d). The speed reduction ratio exerted by the HST, which corresponds to the ratio of capacity of a hydraulic motor to that of a hydraulic pump in the HST, is conventionally changed by a driver""s manual operation of a movable swash plate of the variable capacity hydraulic pump. However, the conventional IHT used in such a working vehicle is not provided with a device automatically changing the speed reduction ratio of its HST in correspondence to output of or load on an engine.
Incidentally, even if the speed reduction ratio of such an HST in IHT for a working vehicle can be automatically changed correspondingly to load on an engine, there are still some cases such that the automatic changing of the speed reduction ratio is not desired. Particularly, if the working vehicle is a lawn tractor, it is preferable that, during its mowing work, the speed reduction ratio is not automatically changed, but manually changed only by a driver""s will for preventing the lawn from being mowed unevenly.
A main object of the present invention is to provide a hydrostatic vehicle driving system between an engine and an axle, used in a working vehicle like a lawn tractor, including an HST which comprises a hydraulic pump and a hydraulic motor fluidly connected with each other through a hydraulic circuit, wherein the speed reduction ratio exerted by the driving system can be automatically steplessly changed in correspondence to load on the engine without the use of a conventional belt transmission having a split pulley for minimization and cost-saving of the vehicle.
To achieve the object, the hydraulic motor is made to be a variable displacement hydraulic motor, whose capacity is controlled by a motor capacity control system in correspondence to the condition of load on the engine.
Consequently, the speed reduction ratio of the HST is automatically optimally increased for avoiding overload on the engine, and reduced for effectively transmitting power of the engine to the axle. The present vehicle driving system with an HST can be improved in its minimization and cost-saving because there is no use of the belt transmission as mentioned above.
Preferably, the hydraulic pump, the hydraulic motor and the axle are contained in a housing, thereby constituting a compactly integrated transaxle apparatus.
The motor capacity control system comprises load-detection means detecting hydraulic pressure in the hydraulic circuit corresponding to the load on the engine, a hydraulic actuator for changing the capacity of the hydraulic motor, and actuator-control means controlling the hydraulic actuator according to the hydraulic pressure detected by the load-detection means.
Accordingly, the capacity of the hydraulic motor can be changed without an expensive electric sensor or actuator, but with hydraulic oil utilized as it is used in the HST or the transaxle apparatus, thereby enabling the motor capacity control system to be provided simply and at low cost.
If there are some cases that the vehicle having the above-mentioned present hydrostatic vehicle driving system is desired to cruise at a constant speed, the automatically controlled capacity of the hydraulic motor is preferred to be fixed.
Then, the present hydrostatic vehicle driving system comprises a manual mode selection member which is provided on a working vehicle so as to be switched between a first mode position and a second mode position. When the mode selection member is located at the first mode position, the capacity of the hydraulic motor is fixed, and when the mode selection member is located at the second mode position, the capacity of the hydraulic motor can be varied by the motor capacity control system.
If the hydraulic motor is of an axial piston type, the hydraulic motor is provided with a movable motor swash plate having a contact surface abutting against a piston of the hydraulic motor. The motor swash plate is moved from a first angle to a second angle larger than the first angle accordingly to increase the load detected by the load detection means. The first angle and the second angle are respectively formed between the contact surface of the motor swash plate and the phantom plane perpendicular to a rotary axis of the hydraulic motor.
When the motor swash plate is located at the first angle, the capacity of the hydraulic motor is smaller so as to establish the smaller speed reduction ratio of the HST. When the motor swash plate is located at the second angle, the capacity of the hydraulic motor is greater so as to establish the greater speed reduction ratio of the HST.
Particularly, if the vehicle is a lawn tractor, it is preferable that the first mode position corresponds to its traveling during lawn-mowing and the second mode position corresponds to its regular traveling on a road or the like. If the variation of capacity of the hydraulic pump is out of consideration, then when the vehicle travels for mowing, the mode selection member is located at the first mode position so as to fix the speed reduction ratio, thereby enabling the vehicle to cruise at a constant speed so as to prevent the lawn from being mowed unevenly. When the vehicle travels on a road or the like out of mowing-work, the mode selection member is located at the second mode position so as to change the speed reduction ratio in correspondence to the detected load on the engine, thereby enabling an effective speed control and prevention of overload on the engine.
The hydraulic pump is a variable displacement hydraulic pump, and a manual speed control member is provided on the working vehicle for changing capacity of the hydraulic pump.
The capacity of the hydraulic pump is controlled by operation of the speed control member whether the mode selection member is located at the first mode position or the second mode position. In other words, when the mode selection member is located at the first mode position, the speed reduction ratio is not automatically controlled by the motor capacity control means, but manually changed only by operation of the speed control means. When the mode selection member is located at the second mode position, the speed reduction ratio is automatically controlled by the motor capacity control means in addition to its manual change by operation of the speed control means.
If the hydraulic pump is an axial piston type variable displacement hydraulic pump, the manual speed control member is operated so as to move a movable pump swash plate of the hydraulic pump.
The speed control member may be selectively connected to a carburetor of the engine so that, when the mode selection member is located at the second mode position, the speed control member is operated so as to control both capacity of the hydraulic pump and output rotary speed of the engine. Especially, the speed control member may be selectively connected to a throttle member of the carburetor so that, when the mode selection member is located at the second mode position, the speed control member is operated so as to move both the pump swash plate and the throttle member.
Therefore, on the above-mentioned assumption that the vehicle is a lawn tractor, during its regular traveling out of mowing work, only the speed control member is manipulated without manipulation of an accelerator member provided on the vehicle, thereby enabling the engine and transmission in the vehicle to be controlled with one hand. If the vehicle is to ascend a slope or start while the mode selection member is located at the second mode position, the engine is desirably accelerated in association with the shift of the transmission to highspeed by such an easy operation.
For an alternative mechanism for capacity control of the variable displacement hydraulic pump in association with output control of the engine, an actuator for changing capacity of the hydraulic pump may be provided in addition to the manual speed control member so as to be controlled according to the output of the engine, and a selection means is provided for selecting one of the manual speed control member and the actuator so as to change the capacity of the hydraulic pump. This selection means may be provided in association with the mode selection member so that, while the selection means selects the speed control member, capacity of the hydraulic motor is fixed, and that, while the selection means selects the actuator, capacity of the hydraulic motor can be changed.
In this regard, the above-mentioned hydraulic circuit for fluidly connecting the hydraulic pump and the hydraulic motor with each other serves as a first hydraulic circuit for driving the hydraulic motor, and additionally, a second hydraulic circuit is extended from a discharge port of a charge pump driven by the engine for supplying the first hydraulic circuit with fluid. An orifice is provided on the way of the second hydraulic circuit, so that the actuator is driven according to difference of hydraulic pressure in the second hydraulic circuit between upstream and downstream of the orifice.
Consequently, when the selection means selects the actuator, capacity of the hydraulic pump is varied according to operation of a manual accelerator member such as a pedal or a lever for adjusting the throttle of a carburetor of the engine, thereby enabling both engine controlling and transmission shifting with one hand (foot).
Other and further objects, features and advantages of the present invention will appear more fully from the following description.